An aircraft is moved by a number of jet engines each housed in a nacelle which also accommodates a set of auxiliary actuating devices which are associated with the operation of said aircraft and which perform various functions when the jet engine is operating or at a standstill. These auxiliary actuating devices particularly comprise a mechanical thrust reversal system.
A nacelle generally has a tubular structure comprising an air inlet upstream of the jet engine, a mid-section intended to surround a fan of the jet engine, and a downstream section accommodating the thrust reversal means and intended to surround the combustion chamber of the jet engine, and is generally terminated by an exhaust nozzle whose outlet is situated downstream of the jet engine.
Modern nacelles are intended to accommodate a bypass jet engine, or turbofan, which is capable of generating, via the rotating blades of the fan, a hot airflow (also termed primary flow) emanating from the combustion chamber of the turbofan, and a cold airflow (secondary flow) which flows outside the turbofan through an annular passage, also termed duct, formed between a cowling of the turbofan and an internal wall of the nacelle. The two airflows are expelled from the turbofan through the rear of the nacelle.
The job of a thrust reverser is to improve the braking capability of an aircraft while it is landing by forwardly redirecting at least a fraction of the thrust generated by the turbofan. In this phase, the reverser obstructs the duct for the cold flow and directs this cold flow toward the front of the nacelle, thereby generating a counter-thrust which combines with the braking of the aircraft wheels.
The means employed to achieve this reorientation of the cold flow vary according to the type of reverser. However, in all cases, the structure of a reverser comprises movable cowls which can be shifted between, on the one hand, a deployed position in which they open in the nacelle a passage intended for the deflected flow, and, on the other hand, a stowed position in which they close this passage. These cowls can perform a deflection function or simply a function of activating other deflection means.
In the case of a grid-type reverser, also known by the name of a cascade-type reverser, the airflow is reoriented by deflection grids, the cowl having only a simple sliding function with the aim of uncovering or covering these grids. Complementary blocker doors, also termed flaps, activated by the sliding movement of the cowling, generally make it possible to close off the duct downstream of the grids so as to optimize the reorientation of the cold flow.
These flaps are pivotally mounted, by an upstream end, to the sliding cowl between a refracted position, in which, together with said movable cowl, they provide aerodynamic continuity of the internal wall of the nacelle, and a deployed position in which, in a thrust reversal situation, they at least partially block off the annular duct in order to deflect a gas flow toward the deflection grids uncovered by the sliding movement of the movable cowl. The pivoting movement of the flaps is guided by means of links which are attached, on the one hand, to the flap and, on the other hand, to a fixed point of the internal structure defining the annular duct.
A first problem with such a configuration concerns the kinetics of the degree of opening of the flaps which, at the start of the opening phase of the movable cowls, is quicker than the opening of said cowl. The consequence of this is that, at the start of the opening phase of the movable cowls, the passage section across the nacelle is smaller than the section of the duct which is blocked by the flaps. This results in an increase in the pressure in the engine, thereby entailing difficult management of the turbofan speed in this transient phase.
A second problem concerns the guide links passing through the duct and thereby causing numerous aerodynamic disturbances in the secondary flow.
Fastening the links to the internal structure constitutes a third problem. Specifically, the installation of fixed hinge points reduces the area of the internal structure that can be used for acoustically treating said internal structure.
Finally, a fourth problem concerns the fact that the thrust reverser structure is mechanically connected by the links to the internal structure. As a result, the thrust reverser structure and the internal structure are not independent of one another, thus complicating their removal when maintenance operations on the nacelle or the turbofan make this necessary. It should be pointed out that this problem more particularly concerns internal structures of the so-called “O-duct” type, that is to say produced from a single piece completely surrounding the turbofan, as opposed to the structures of the “C-duct” type comprising two half-portions joined together around the turbofan.
A number of solutions have been adopted to solve one or more of these problems.
Document U.S. Pat. No. 3,262,268, for example, describes such a grid-type thrust reverser in which a linkage for controlling the pivoting movement of the flap comprises two “scissors” levers, one lever of which is hinged on the sliding cowl and the other lever of which, which is more downstream, is hinged on guide beams belonging to the external nacelle.
This solution avoids the use of connecting links between the flap and the internal structure.
However, the scissors-type linkage, which is simple and lightweight, has the disadvantage of deploying the flap very quickly in the annular duct at the start of the retreating travel of the sliding cowl, and therefore does not solve the problem of the difference in opening kinetics between the movable cowl and the flaps.
Document U.S. Pat. No. 4,005,822 also describes such a thrust reverser in which the flaps are pivotally mounted on the movable cowl and attached to a link mounted on the actuating means of the movable cowl in such a way that, once the actuating means are at the end of their travel, they cause the link to retreat, thus pivoting the flap in the process.
Such a system allows a delayed opening of the flaps with respect to the opening of the movable cowl, thus preventing an increase in pressure in the duct. However, the converse disadvantage arises, since the passage section across the nacelle, added to those of the two flows in direct jet mode, is too large with respect to the air inlet section of the nacelle. Such a situation is also prejudicial to the turbofan.
It will also be noted that the grids are integrated with a movable ring section which is moved together with the movable cowl, said ring section being a bulky element and having an impact on the mass of the nacelle assembly. The presence of this movable ring section also requires dedicated guide elements which have an impact on the mass of the assembly and complicate the use of the system.
Finally, it will be noted that the screw used to drive the flaps is directly exposed to the aerodynamic pressure forces exerted on the flaps, with the risk of a resulting deformation which is incompatible with the reliability required for such a system.
Finally, mention will be made of document U.S. Pat. No. 4,909,442 which provides a complex drive system by means of hydraulic or pneumatic cylinders attached to the movable cowl and to the flap using a set of communicating vessels.